(1) Field of the Invention
The present invention relates to a noise suppression apparatus for suppressing noise of a luminous signal in a television receiver at, for example, a fringe area of reception.
(2) Description of the Prior Art
FIG. 8 is a block diagram of a noise suppression apparatus proposed in Japanese Patent Laid-Open 2-213284. In FIG. 8, the block 1 is a tuner. The block 2 is an IF processing circuit. The block 3 is a feedback type noise reducer. The block 4 is a video signal processing circuit. The block 5 is a display. The block 6 is a tuning circuit. The block 7 is a feedback coefficient control circuit. The voltage 8 is a tuning voltage. The voltage 9 is an AGC voltage. The signal 10 is a video signal.
The function of a noise suppression apparatus composed like the above is explained below.
When an electric field strength is strong, if an AGC circuit of tuner 1 is a forward type, RF AGC voltage 9 is high and output K of feedback coefficient control circuit 7 is small. Therefore, the component ratio of the present frame to the last frame at the output of feedback type noise reducer 3 has a less effect to reduce noise when the present video signal is large but there is no blur at a violent movement scene such as scene changing.
On the other hand, when the electric field strength is weak, RF AGC voltage 9 is low and output K of feedback coefficient control circuit 7 is large. Therefore, the component ratio of the present frame to the last frame at the output of feedback type noise reducer 3 brings a big blur when the video signal output at a frame ahead of the present frame is large and for a violent movement scene but noise reducing effect becomes large.
However, in the above composition, there are the following problems. FIGS. 9(a) and 9(b) illustrate characteristics of a noise suppression apparatus proposed in Japanese Patent Laid-Open 1-213284. FIG. 9(a) shows a characteristic of RF AGC voltage 9 against electric field strength inputted and FIG. 9(b) shows a characteristic of output voltage of feedback coefficient control circuit 7 against electric field strength inputted.
Generally, when the input electric field strength is smaller than a certain level (point A in FIGS. 9(a) and 9(b)), the RE AGC voltage does not change. Because also the output voltage of feedback coefficient control circuit 7 similarly changes, if the input electric field strength is smaller than a certain level (point A in FIGS. 9(a) and 9(b)), the noise reduction effect does not change.
Considering that the noise remarkably increases when the input electric field strength is smaller than a certain level (point A in FIGS. 9(a) and 9(b)), the noise reduction effect at a fringe area reception can not be so expected.